The <i>IgH</i> Locus of the Channel Catfish, <i>Ictalurus punctatus</i>, Contains Multiple Constant Region Gene Sequences: Different Genes Encode Heavy Chains of Membrane and Secreted IgD

Abstract

The delta-chain of catfish IgD was initially characterized as a unique chimeric molecule containing a rearranged VDJ spliced to C micro 1, seven C domain-encoding exons (delta1-delta7), and a transmembrane tail. The presence of cDNA forms showing splicing of delta7 to an exon encoding a secretory tail was interpreted to indicate that membrane (deltam) and secreted (deltas) forms were likely expressed from a single gene by alternative RNA processing. Subsequent cloning and sequence analyses have unexpectedly revealed the presence of three delta C region genes, each linked to a micro gene or pseudogene. The first (IGHD1) is located 1.6 kb 3' of the functional C micro (IGHM1). The second (IGHD3) is positioned immediately downstream of a pseudo C micro (IGHM3P), approximately 725 kb 5' of IGHM1. These two delta genes are highly similar in sequence and each contains a tandem duplication of delta2-delta3-delta4. However, IGHD1 has a terminal exon encoding the transmembrane region, whereas IGHD3 has a single terminal exon encoding a secreted tail. The occurrence of IGHD3 immediately downstream of a micro pseudogene indicates that the putative deltas product may not be expressed as a chimeric micro delta molecule. Western blots and protein sequencing data indicate that an IGHD3-encoded protein is expressed in catfish serum. Thus, catfish deltam transcripts appear to originate from IGHD1, whereas deltas transcripts originate from IGHD3 rather than, as previously inferred, from a single expressed delta gene. The third delta (IGHD2) is associated with a pseudo C micro (IGHM2P); its presence is inferred by Southern blot analyses.